Some CommonMark.dtd inconsistencies with cmark XML output

[gemmaro]

While comparing the CommonMark.dtd schema with the XML output produced by cmark, I noticed a couple of mismatches. It's unclear whether the DTD or the implementation should be updated, but these differences may cause validation issues.

1. delim attribute of list element: DTD uses delimiter while the implementation emits delim
2. xml:space attribute of text element: no attributes defined in DTD while the implementation emits xml:space

There is an exisiting pull request related to the DTD at #541, but these specific discrepancies may not be covered there.

Thank you.